On a recent Saturday afternoon in a quiet suburb north of Seattle, Wash., Will Peterman leaned into a thick hedge of blackberry bushes, looking for a lost species. The hedge buzzed with bees of every description -- in a single glance, Peterman counted six species -- but two plump, slow-moving bumblebees, each about the size of a small grape, captured his attention.

"There's one," he said quietly. "And there's another one."

They were Western bumblebees, a native species. Bombus occidentalis was once among the most common bumblebees in the Western United States. In the late 1990s, its populations crashed, and the species all but disappeared from about a quarter of its historic range. Despite regular surveys of the Seattle area by researchers and volunteers, no sighting was confirmed for well over a decade. Then, in early July, Peterman confirmed a 2012 sighting in this neighborhood.

In early September, biologists from the University of Colorado at Boulder announced that Western bumblebees had also returned to Colorado's Front Range, with more than 20 individuals observed during surveys in 2012 and 2013.

Western bumblebees, which usually have a distinctive white patch on their tail ends -- hence their other common name, white-bottomed bees -- are key pollinators of cranberries, blueberries, cherries, greenhouse tomatoes and other species. Their recent reappearance in Washington and Colorado has surprised and delighted both amateur and professional entomologists, particularly because the species is among several North American bumblebees known to be shrinking in numbers and range. One of them, in fact, Bombus franklini, a related Oregon species known as Franklin's bumblebee, is feared extinct.

Like the better-publicized declines of European honeybees, which were introduced to the United States in the 17th century, researchers suspect that bumblebee declines are caused by a combination of factors, including disease and climate and habitat change. And each species decline may have its own story. The Western bumblebee crash in the 1990s is thought to have been caused by a fungus, Nosema bombi, which may have been introduced to the U.S. by commercially bred bumblebee queens from Europe. Another Nosema species, ceranae, has also been implicated in honeybee declines.

No one is certain why and how the Western bumblebee has reappeared, but it's possible that the species is now developing resistance to the Nosema fungus. "We may have hit the mutation jackpot," says Peterman.

Researchers from the University of Washington and the Xerces Society, an invertebrate conservation group based in Portland, Ore., are now looking for more Western bumblebee populations, and four additional sightings were confirmed in and around Seattle this summer. If the species does make a comeback, says the Xerces Society's Rich Hatfield, it will likely have room to re-establish: The society is working with farmers to plant pollinator-friendly flowers and shrubs in field margins and other fallow lands, an effort that's created about 100,000 acres of pollinator habitat nationwide. And some species of native bumblebees are still doing well along the West Coast, suggesting that at least some suitable Western bumblebee habitat persists.

Farmers value bumblebees because they use their tiny jaws to bite and shake each flower, a distinctive and efficient pollen-dislodging practice known as "buzz pollination." Humans can mimic buzz pollination with various gadgets, including electric toothbrushes, but the process is laborious and prohibitively expensive on a large scale. Private companies have been trying to breed Bombus occidentalis in captivity, says Jim Strange of Utah State University, but have struggled to protect bees from fungal infection. Since the Western bumblebee population crash, some farmers of bumblebee-reliant crops have resorted to honeybees, which can manage the job but aren't as effective, or have purchased bees from a related Eastern species, Bombus impatiens.

But many conservationists oppose the introduction of non-native bumblebees into the region, fearing the spread of new diseases. In 2010, the Xerces Society and other conservation groups petitioned the U.S. Department of Agriculture to create regulations preventing the movement of bumblebees outside their native ranges. While the federal government did not act on the petition, in 2012 the Oregon Department of Agriculture formalized a longstanding informal ban on the importation of exotic bumblebees into the state. California allows the use of non-native bumblebees in greenhouses, but not in open fields.

If the Western bumblebee is in fact developing Nosema resistance, the species may someday repopulate its former range -- and Western farmers may once again be able to purchase native bumblebees for their greenhouse tomatoes and blackberry fields. For now, though, bee enthusiasts like Will Peterman can only keep an eye on the hedges, looking for another white tail end.